1. Field of the Invention
This invention relates generally to data processing systems and more specifically to a writeable control store for use in the processor of a data processing system.
2. Description of the Prior Art
In many data processing systems, and especially in smaller, economical data processing systems, the internal operations of the processor are controlled by the execution of instructions stored in an internal control store. In such systems, these instructions are typically stored in a read only memory (ROM) of a limited storage capacity. These ROMs are typically programmed at the factory and are not alterable in the field.
However, many systems are provided with additional control store capabilities typically in the form of an option which may be programmed in the field for a particular application. These control stores are typically comprised of a random access memory (RAM) which may be loaded or written with data used to control the operation of the processor.
Typically, the loading or writing of such a control store is controlled by either the main control store (ROM) or the extendable or writeable control store itself. Both of these methods are uneconomical and may degrade the performance of the processor by utilizing additional time to complete the actual loading of the writeable control store.
Utilization of the primary control store or ROM to control data transfers to and from the writeable control store requires the use of additional storage locations within the ROM which must be dedicated for this purpose. In a smaller economical system, these additional storage locations may not be available or may require the sacrifice of additional capabilities of the machine. This is a particularly difficult burden because these storage locations may not even be utilized, since the variable control store is typically provided as an option and, may not be used in many cases. Additionally, the length of the instruction required to control the data transfers to and from the writeable control store will typically be substantially less in size than the length of the instruction normally utilized to control the operation of the machine. Thus, by using the primary control store, storage locations for substantially longer words must be used even though the capacity of each is only partially utilized.
Utilization of the writeable control store to control data transfers to and from itself is uneconomical for the same reasons specified above for the primary control store ROM. Additionally, in most systems, this will also degrade the performance of the machine in that at least one additional machine cycle will be required to complete the loading of the WCS unit. Specifically, a machine cycle will be required to access the instruction utilized to control the transfer and an additional machine cycle will be required to actually complete the loading or writing operation.